<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="js/vue.js"></script>
</head>
<body>
    <div id="app-9">
        <input type="text" v-model="message1"><br />
        <input type="text" v-model="message2"><br />
        {{message3}}
    </div>


    <script type="text/javascript">
        var vm = new Vue({
            el: '#app-9',
            data: {
                message1: 'Hello',
                message2: 'World'
            },
            computed: {
                message3: function () {
                    return this.message1 + this.message2;
                }
            }
        })

    </script>
</body>
</html>